On the security of some nonrepudiable threshold proxy signature schemes with known signers.

A (t, n) threshold proxy signature scheme enables an original signer to delegate the signature authority to a proxy group of n member such that t or more than t proxy signers can cooperatively sign messages on behalf of the original signer. In the paper, we review the security of some nonrepudiable threshold proxy signature schemes with known signers. We show that Sun's threshold proxy scheme, Yang et al.'s threshold proxy signature scheme and Tzeng et al.'s threshold proxy signature scheme are insecure against an original signer's forgery. We also show that Hsu et al.'s threshold proxy signature scheme su#ers from the conspiracy of the original signer and the secret share dealer SA, and that Hwang et al.'s threshold proxy signature scheme is universally forgeable. In a word, none of the above-mentioned threshold proxy signature schemes can provide non-repudiation.